1. Field of the Invention
The present invention relates to an information recording medium on which it is possible to record information as a magnetic mark by applying light and an external magnetic field to the medium, and from which it is possible to reproduce the recorded information by detecting the magnetic leakage field leaking from the magnetic mark. This invention also relates to an apparatus for recording information on and reproducing information from such a recording medium. The invention further relates to a method for positioning a head of such a recording and reproducing apparatus. More specifically, the invention relates to an information recording medium which can prevent the head crashes caused by the contact of the recording head or the reproducing head of a recording and reproducing apparatus with the recording medium, and to a novel recording and reproducing apparatus and a novel head positioning method which can reliably position the recording and reproducing heads at target code tracks of an information recording medium.
2. Description of Related Art
Magneto-optical disks and other magneto-optical recording media are known as external memories for computers etc. It is possible to rewrite information and record a great quantity of data such as voice and dynamic images on magneto-optical recording media. Therefore, magneto-optical recording media are used widely as recording media in the multimedia age. The recordation of information on a magneto-optical recording medium involves condensing a laser beam on the recording medium through a converging lens to form a light spot on the medium, in order to heat the recording medium locally by means of the light spot, and applying a magnetic field corresponding to the information to the heated region to record an information mark on a code track (information track) of the recording medium. The reproduction of information from a magneto-optical recording medium involves irradiating an information mark region of the recording medium with a laser beam lower in laser power than the laser beam for recordation, and detecting a magneto-optical effect depending on whether an information mark is present or absent in the region.
For higher recording density of a magneto-optical recording medium, its code tracks may have a narrower track pitch, and information marks may be lined at a narrower mark pitch in the direction of the scanning of the recording medium with a light spot. There may be a case where the track pitch and/or the mark pitch are/is smaller than the diameter of the light spot. In this case, however, at the same time that one of the information marks is irradiated with the light spot, one or more of the adjacent information marks are partially irradiated. Consequently, the signal or signals reproduced from the adjacent wrong mark or marks leak into the signal reproduced from the right mark. The leakage interferes as one or more noise components with the signal reproduced from the right mark, and consequently lowers the reproducing space resolution. This prevents information from being accurately reproduced from the recording medium. Accordingly, the spot diameter limits the track and mark pitches.
If a converging lens having a numerical aperture NA forms a light spot of a laser beam having a wavelengthλ, the diameter Ws of the spot is represented generally by the valueλ/NA. For example, even if a solid immersion lens (effective NA=1.2) is used with a blue light source (λ=413 nm), the spot diameter Ws is about 0.34 μm. There may be a case where, with the track pitch narrowed, minute marks are recorded at a pitch of 0.09 or less μm, which is about a quarter of the spot diameter Ws, in the direction along the code tracks. If a signal is reproduced from any one of the recorded marks on any one of the code tracks with a light spot having a diameter of 0.34 μm, the signal or signals from one or more of the preceding and succeeding marks on this track leak into that signal, which can therefore not be distinguished.
Even if a current magnetic disk apparatus forms minute information marks at a pitch of about 0.07 μm on a magnetic disk, it is possible to reproduce signals having sufficient amplitude from the marks, because the apparatus includes a reproducing magnetic head which is high in space resolution. Thus, in an optical disk apparatus fitted with a laser which is specific in wavelength and a converging lens, the spot diameter, on which the space resolution for reproduction depends, is considerably larger than a minute information mark. Therefore, this optical disk apparatus is low in reproducing resolution. This is a bar to the densification of optical disks.
A method proposed for solving this problem is the magneto-optical fusional system described in “Proceedings of Magneto-Optical Recording International Symposium '99”, pp. 225–228. This system employs a magneto-optical recording medium as an optical recording medium. As is the case with conventional optical disk apparatuses, the recordation of information on a magneto-optical recording medium involves scanning the recording medium with a laser beam radiated by an optical head and a magnetic field applied by a magnetic head. This forms, on a code track of the recording medium, an information mark represented by one of the directions of magnetization which are perpendicular to the surfaces of the recording medium.
Information can be reproduced from the magneto-optical recording medium by a reproducing means which is similar to conventional magnetic disk apparatuses. The reproducing means includes a magnetic head on which a magneto-resistance element is mounted. The information recorded on the recording medium can be reproduced by the magnetic head detecting the magnetic leakage field leaking from the information mark recorded on the recording medium. The magneto-resistance element has a width of some tens of nanometers in the directions along the code tracks of the recording medium. This width is small in comparison with the foregoing light spot diameter, which is hundreds of nanometers. Therefore, the reproduction with the magneto-resistance element can improve the reproducing resolution in the directions along the tracks. It is also possible to improve the reproducing resolution in the directions across the code tracks by narrowing the magneto-resistance element in these directions. Thus, the magneto-optical fusional system solves the problem of information leakage with the conventional optical disk apparatuses by detecting information during reproduction with the magnetic head on which the magneto-resistance element is mounted. This allows the narrowed track pitch and mark pitch to make the recording density higher.
The magneto-optical fusional system causes the magnetic heads to track or follow the code tracks by a method disclosed in Japanese Patent Application Laid-Open No. 2-218016. This method includes forming predetermined regions of a magneto-optical recording medium with pits which have a magnetically readable structure, causing a magnetic head to detect, as tracking signals, the magnetic leakage fields leaking from the ends of the pits, and controlling the position of the magnetic head on the basis of the detected tracking signals. In order for an optical head to track a code track, the position of the optical head is controlled by the sample servo tracking system employing the pits and used with the conventional optical disk apparatuses.
As stated above, a magneto-optical recording medium used with the magneto-optical fusional system has pits formed in its predetermined regions so that a magnetic head and an optical head can track the code tracks of the recording medium. However, while the floating slider on which the magnetic and optical heads are mounted is scanning over the regions where the pits are formed, the slider may contact the recording medium and cause a head crash. The preset floating height of the slider of each conventional recording and reproducing apparatus on this system is large enough to cause no head crash even if the floating height lowers over the regions of a magneto-optical recording medium which are formed with pits.
The magnetic leakage fields leaking from information marks micrified for higher density are smaller than those leaking from information marks recorded at lower density. Therefore, in order to reproduce densely recorded information from a recording medium, it is demanded that the floating height of the slider of a recording and reproducing apparatus be as small as possible so that the small magnetic leakage fields leaking from minute information marks on the medium can be detected reliably or securely. As stated above, however, if the preset floating height of the slider of a recording and reproducing apparatus on the magneto-optical fusional system is small, head crashes are liable to occur in the regions of a recording medium which are formed with pits. It is consequently difficult to reliably reproduce the densely recorded minute marks. If a magneto-optical recording medium is formed with recessed or pitted and embossed patterns in and on a surface thereof, dust etc. may collect in the recesses of the patterns and be another main cause of head crashes.